Printing on sheets of substrate media by direct marking is a rapidly expanding marking technology due to low run-costs and overall simplicity. Direct marking printing includes printing systems using inkjet technology where one or more print heads are located proximate to the sheet surface. As resolutions improve, many believe that direct marking will make inroads relative to markets where xerographic systems currently dominate. Three challenges with direct to paper marking systems include; achieving good marking quality of the media, holding the media away from the print-heads to prevent burnishing or clogging of the print head nozzles, and achieving sufficiently high resolution with a single pass at low costs.
In such systems it is important to consistently hold the sheets flat as they pass by the print heads. If the portion of the sheet onto which an image is to be printed is not flat, the image quality will suffer. Moreover, if the edges or any part of the sheet project upwardly, they can engage the print heads causing damage. In order to hold the sheet flat, media vacuum hold-down drums or plates have been used. Such drums/plates typically enable good marking quality and enable multi-pass printing which requires fewer print heads and saves cost.
However, a drum/plate increases the challenge of holding the media away from the print heads with upcurled sheet leading and trailing edges becoming especially challenging. As shown in FIG. 1, in prior art systems with vacuum drum 4 having a smooth drum surface 6, the bending moment exerted by the vacuum on the sheet 8 becomes very small as you get closer to the edge of the media 10 (and eventually becomes zero). This makes it very difficult to hold the edge tightly to the drum and a gap 12 can exist.
Accordingly, it would be desirable to provide a media hold-down apparatus and system which improves hold down performance of the leading and trailing edges of substrate media.